1. Field of the Invention
The present invention relates to a cathode-ray tube in which electron beams emitted from an electron gun are deflected to project an image on a fluorescent screen and also relates to an electron gun used in the cathode-ray tube.
2. Description of the Related Art
FIG. 1 is a schematic cross sectional view showing a conventional color cathode-ray tube provided with an electron gun. In the figure, reference numeral 31 indicates a glass enclosure which has an ordinary cathode-ray tube shape and includes a neck 32, a funnel 33 and a face 34. An electron gun 24 is arranged at the neck 32 of the glass enclosure 31. Fluorescent materials for red, green and blue are applied in a mosaic fashion to an inner surface of a face plate 27 of the face 34 to form a fluorescent layer 26. An inner duck 23 for allowing conduction of a high voltage is formed at an inner surface of the funnel 33. A deflection yoke 22 is arranged around a junction between the funnel 33 and the neck 32 so as to deflect the electron beams emitted from the electron gun 24.
The electron gun 24 includes a cathode 1 for emitting electrons, a control electrode 2 for controlling a path of the electron beams emitted from the cathode 1, an accelerating electrode 3 for accelerating the electron beams, a focusing electrode 35 for focusing the electron beams, and a final accelerating electrode 8 for finally accelerating the electron beams. The final accelerating electrode 8 is conductively welded to a shield cup 36, and is applied with a high voltage from an anode button via the inner duck 23 and the shield cup 36. Predetermined voltages are applied to the other electrodes (i.e., control electrode 2, accelerating electrode 3 and focusing electrode 35) via pins 37 arranged at an end of the neck 32. The cathode 1 is formed of a cathode 1a for a red beam, a cathode 1b for a green beam and a cathode 1c for a blue beam.
According to the cathode-ray tube thus constructed, the electron beams emitted from the electron gun 24 are deflected by the deflection yoke 22 and impinged on the fluorescent layer 26 to form a visible image.
The resolution characteristics of the cathode-ray tube depend significantly on a diameter and a shape of a spot of the electron beams impinging on the fluorescent layer 26. More specifically, as the spot diameter decreases and/or the spot roundness increases, the resolution is improved. The small diameter and high roundaness of the spot are required of the screen of the fluorescent layer 26 on which the image is formed.
However, the screen surface (fluorescent layer 26 and face plate 27) of the cathode-ray tube is generally flat, so that, to a higher extent the electron beam is deflected toward a periphery of the screen, a longer distance the electric beam travels. Therefore, when the focus voltage provided to the focusing electrode 35 is controlled to reduce the spot diameter of the electron beams at a central portion of the screen and to increase the roundness of the same, the electron beams at the peripheral portion of the screen are over-focused and thus cannot form the electron beam spot of a small diameter. This results in reduction of the resolution.
In view of the above, a so-called dynamic focus system has been proposed. In this system, the focus voltage applied to the focusing electrode 35 is increased to -weaken the focusing function of the main electron lens formed of the focusing electrode 35 in accordance with an increase in the degree of deflection of the electron beams. However, this system is not suitable to a self-convergence system used in an in-line type electron gun which has generally been used in recent years, for the following reasons. The inline type electron gun emitting three electron beams along a horizontal straight line, employs the self-convergence system in which the horizontal deflection magnetic field is distorted in a pin-cushion-like form, and a vertical deflection magnetic field is irregularly distorted in a barrel-like form. The electron beams passing therethrough are forced to diverge horizontally and to converge vertically, so that they form a horizontally long flat form.
Accordingly, the electron beams deflected toward the periphery of the screen have a horizontal spot diameter maintained at the optimum focus state, but they have a vertical spot diameter in the over-focus state, resulting in generation of a low luminance portion, i.e., so-called "halo". In connection with this state, when the dynamic focus system is employed, the vertical spot diameter in the over-focus state is corrected, so that generation of the low luminance portion, i.e., halo, is avoided and the optimum focus state is attained. Meanwhile, the horizontal spot diameter is out of the optimum focus state and is in the under-focus state. Thereby, the horizontal spot diameter increases, and the horizontal resolution is remarkably impaired. Accordingly, the resolution at the periphery of the screen is not improved.
FIG. 2 is an enlarged cross sectional view of the conventional electron gun 24 for overcoming the above problems, which is disclosed in Japanese Patent Application Laid-Open No. 3-93135 (1991). This electron gun 24 includes the cathodes 1a, 1b and 1c, control electrode 2, accelerating electrode 3, first auxiliary electrode 4, second auxiliary electrode 5, first focusing electrode 6, second focusing electrode 7 and final accelerating electrode 8 which are disposed in this order. This conventional electron gun has been called an electron gun of a bipotential type. Structures other than the above are same as those shown in FIG. 1.
FIG. 3 is a front view of the first auxiliary electrode 4. The first auxiliary electrode 4 has a plate-like form, and is provided at positions corresponding to the cathodes 1a, 1b and 1c with circular apertures 4a, 4b and 4c. On a surface of the first auxiliary electrode 4 near to the screen, there are provided paired plates 4d, 4e and 4f which are located at vertically opposite sides of the apertures 4a, 4b and 4c, respectively, and each has a predetermined thickness and is slightly larger than the diameter of the apertures 4a, 4b and 4c.
FIG. 4 is a rear view from the side of the cathodes 1a, 1b and 1c of the second auxiliary electrode 5. The second auxiliary electrode 5 has a plate-like form, and has circular apertures 5a, 5b and 5c located at positions corresponding to the cathodes 1a, 1b and 1c. On the side of the cathodes 1a, 1b and 1c of the second auxiliary electrode 5, there are provided paired plates 5d, 5e and 5f, which are located at laterally opposite sides of the apertures 5a, 5b and 5c, respectively, and each has a predetermined thickness and is slightly larger than the diameter of the apertures 5a, 5b and 5c. Likewise, on the screen side of the second auxiliary electrode 5 plates 5g, 5h and 5i are set up.
FIG. 5 is a rear view from the side of the cathodes 1a, 1b and 1c of the first focusing electrode 6. The first focusing electrode 6 has a box-like form. On the surface of the cathodes 1a, 1b and 1c side of the first focusing electrode 6, there are formed circular apertures 6a, 6b and 6c located at positions corresponding to the cathodes 1a, 1b and 1c, respectively. On the same surface, paired plates 6d, 6e and 6f are set up, which are located at vertically opposite sides of the apertures 6a, 6b and 6c, respectively, and each has a predetermined thickness and is slightly larger than the diameter of the apertures 6a, 6b and 6c.
FIG. 6 is a front view from the side of the screen of the first focusing electrode 6. On the surface of the screen side of the first focusing electrode, there are formed square apertures 6g, 6h and 6i each having a width slightly larger than the diameter of the apertures 6a, 6b and 6c and a vertical length larger than the width.
FIG. 7 is a rear view from the side of the cathodes 1a, 1b and 1c of the second focusing electrode 7. The second focusing electrode 7 has a box-like form. On the surface of the cathodes 1a, 1b and 1c side of the second focusing electrode 7, there are formed square apertures 7a, 7b and 7c each having a vertical length slightly larger than the diameter of the apertures 6a, 6b and 6c and a lateral width larger than the vertical length.
The first auxiliary electrode 4 and first focusing electrode 6 are maintained at the same potential by a connector, and are applied with a constant focus voltage V.sub.F. The second auxiliary electrode 5 and second focusing electrode 7 are maintained at the same potential by a connector, and are applied with the constant focus voltage V.sub.F. Further, the second auxiliary electrode 5 and second focusing electrode 7 are supplied in a superposed manner with a voltage, which increases in synchronization with a deflecting current as the degree of deflection of the electron beams increases, from a circuit 9.
Operation of the electron gun 24 thus constructed will be described below.
FIGS. 8 and 9 are cross sectional views showing the electron beams and optical operation of electron lenses in the case where the electron beams are deflected toward a periphery of the screen. More specifically, FIG. 8 shows a horizontal section of the electron beams, and FIG. 9 shows a vertical section thereof. In these figures, reference numeral 10 indicates a cross-over position of the electron beams corresponding to an object point. In the figures, numeral 11 shows a path of the electron beams at the most external angle.
A description will now be given in connection with the horizontal direction. The electron beams emitted at a divergence angle .theta. from the object point 10 impinge at an incident angle .theta..sub.H fron the screen (fluorescent layer) 26 after passing through a concave lens 12 which is a horizontal operation of a first quadrupole lens formed of the first auxiliary electrode 4, second auxiliary electrode 5 and first focusing electrode 6, a convex lens 13 which is a horizontal operation of a second quadrupole lens formed of the first focusing electrode 6 and second focusing electrode 7, a convex lens 14 which is a main electron lens formed of the second focusing electrode 7 and final accelerating electrode 8, and a concave lens 15 which is formed by a magnetic field (horizontal component) of the deflection yoke 22.
A description will now be given in connection with the vertical direction. The electron beams emitted at a divergence angle .theta. from the object point 10 impinge at an incident angle .theta..sub.V on the screen (fluorescent layer) 26 after passing through a convex lens 16 which is a vertical operation of the first quadrupole lens formed of the first auxiliary electrode 4, second auxiliary electrode 5 and first focusing electrode 6, the concave lens 17 which is a vertical operation of the second quadrupole lens formed of the first focusing electrode 6 and second focusing electrode 7, the convex lens 14 which is the main electron lens formed, of the second focusing electrode 7 and final accelerating electrode 8, and the convex lens 18 which is formed by a vertical component of the magnetic field formed by a magnetic field (vertical component) of the deflection yoke 22.
The irregular magnetic field produced by the deflection yoke 22 is corrected by the first quadrupole lens (12 and 16), second quadrupole lens (13 and 17) and main electron lens 14, so that the optimum focus state of the electron beams is attained in both the horizontal and vertical directions at the peripheral portion of the screen. Owing to the operation of the first quadrupole lens (12 and 16), the electron beams can form the substantially equal incident angles .theta..sub.V and .theta..sub.V in the horizontal and vertical directions with respect to the screen. As a result, the roundness of the spot shape of the electron beams is improved.
The electron gun of the bipotential type described above has a simple structure, but is not provided with a lens for preliminary convergence, so that the electron beam incident on the main electron lens has a large diameter. Since the diameter of electron beam incident on the main electron lens is proportional to an amount (current) of the electron beam, the electron beam which scans a peripheral region having an increased luminance (i.e., high luminance region) has a large diameter. When the electron beam has a large diameter as described above, the focus is impaired due to the influence by the spherical aberration of the main electron lens.
The above problems may be overcome by an electron gun of a multi-stage convergence type, in which a first subsidiary lens 20 and a second subsidiary lens 21 are disposed between the accelerating electrode 3 and the first auxiliary electrode 4 for preliminarily converging the electron beams as shown in FIG. 10. However, this extremely complicates the structure.